A satellite galaxy of our own Milky
Way, which is by far the smallest galaxy known. Willman 1 (Wil 1), discovered
in 2004 by Beth Willman of New York University, orbits 120,000 light-years
from the center of the Milky Way Galaxy.

Measurements made by a team of astronomers led by Nicolas Martin of the
Max Planck Institute for Astronomy in Heidelberg, Germany, and published
in 2007, shown that the stars within Willman 1 are moving at an exceptionally
low velocity implying that this dwarf system has a very small gravitational
pull equivalent to a mass of only about 500,000 times that of the Sun.

Martin and his team caution that it is possible for a heavier galaxy to
mimic a lighter one, for example by having much more dark
matter than one would expect in its outskirts. However, if Willman 1
really is as lightweight as it seems, then it may be only the first of many
such ultra-low-mass galaxies waiting to be found. Hundreds of such systems
could be in orbit around the Milky Way alone.